Elements from the Sea

1. Elements from the Sea Chemical Storylines notes

2. ES1 Why is the sea so salty • Hydrothermal vents are geysers on the sea floor… • They constantly spew out mineral-rich water • They occur where tectonic plates move apart • When they do, sea water enters the crack and gets heated by the mantle • Hot water can dissolve more minerals • These then get carried into the sea when it ‘erupts’ • Black smokers are the hottest vents • The ‘smoke’ they produce is often iron sulphide • This is insoluble and forms by a precipitation reaction… • …other ions such as chloride, bromide and some group 1 and 2 metals are more soluble and so stay in solution

3. CI5.1 “Ions in solids and solutions” • Hydrothermal vents account for much of the dissolved chemicals in the sea • Others will have reached the sea as rainwater has weathered rocks and eventually run into rivers • The constant evaporation of water has caused the salts to accumulate • Most seas have similar concentrations of dissolved minerals but the Dead Sea is different… • CI1.5 “Concentrations of solutions”

4. ES2 The lowest point on Earth • The lowest point on Earth is the Dead Sea(~400m below sea level) • There is no outflow and the surroundings are desert • Over thousands of years the resulting evaporation has led to huge accumulations of salts • The water contains about 350gdm-3 of salts compared to 40gdm-3 in the oceans • It is thought to contain about 43 billion tonnes of salts • It contains about 75 times the bromide ions of ocean water • A large chemical industry has developed as result • Israel is one of the largest bromine producers in the world • CI2.5 “Ionisation” • ES2.1 “Writing the formulae of ionic compounds” • ES2.2 “Solutions of ions” • CI3.1 revisit writing formulae (qq 10, 11)

5. Bromine from sea water • Uses of bromine: • Many pharmaceuticals – analgesics, sedatives, antihistamines • Flame retardants • Extracting bromine from bromide ions in the lab is simple; Br-(aq) + Cl2(aq) Br2(aq) + 2Cl-(aq) • Extracting it from sea water is more complicated (see next slide)… • The sea water is acidified and warmed; • Chlorine (Cl2) is added (same reaction as above); • Steam is blown through the solution; • The vapours are then condensed and bromine forms a separate layer beneath the water (denser than water) • Br2 is then distilled and dried • This is an example of a very important type of reaction with involves one species being oxidised and one being reduced • Such reactions are called redox reactions… • CI9.1 ”Oxidation and Reduction” • CI11.4 ”Group 7 – The halogens” • CI 2.4 “ Sub shells and orbitals” • Assignment 3

6. SEPARATOR Warm solution rich in Br- ions Br2, Cl2 and steam DISTILLATION COLUMN CONDENSER REACTION COLUMN and STEAMING OUT TOWER water Acidic effluent solution Br2 and some Cl2 Cl2 and some Br2 steam chlorine Damp Br2 Dil. sulphuric acid DRYING 98% sulphuric acid Pure Br2

7. ES3 An industrial case study – how best to manufacture chlorine? • Made by electrolysis of concentrated sodium chloride solution (brine) (NaCl (aq)) • Can be extracted from the sea or from rock salt which can be mined or collected by pumping water into it • Electrolysis of brine produces two other very useful chemicals; hydrogen (H2)and sodium hydroxide (NaOH) (an alkali) • The industry is often referred to as the chlor-alkali industry • We will look at two ways of producing chlorine from brine both of which use electrolysis; the mercury cell and the membrane cell • CI15.1-15.6 “Greener Industry” Mercury cell • First large-scale method used; • Uses toxic mercury and is more expensive to run than other methods • Will be phased out in Europe by 2020

8. Membrane Cell • Introduced in 1980s • Improvement on mercury cell because; • Lower energy requirements so lower running costs • More Cl2 can be made in the same factory space • No need for costly removal of mercury • Less pollution • Start up costs (fixed costs) are recouped within 5 years • Half equations are; • At +ve electrode (anode): 2Cl-(aq) Cl2(g) + 2e- • At -ve electrode (cathode): 2H2O(l) + 2e- 2OH-(aq) + H2(g) • Overall… 2Cl-(aq) + 2H2O(l) Cl2(g) + 2OH-(aq) + H2(g)

9. The cell must be designed to: • Keep Cl2 at +ve electrode away from OH- around –ve electrode • Minimise Cl- contaminating NaOH solution • Minimise OH- diffusing towards +ve electrode • Prevent Cl2 and H2 mixing which could lead to an explosion • This is achieved by using a membrane made of poly(tetrafluoroethene) (PTFE or teflon)

10. - - - - - - • The PTFE is modified so it has negatively charged side groups • This means the PTFE is a barrier to gases and liquids and even repels OH- ions • Therefore only sodium ions (Na+(aq)) are able to pass through…… • Assignment 4 and 5 Na+ Na+ Na+ Na+ Na+ Na+

11. ES4 From atomic bombs to drinking water • Although halogens are hazardous to produce and transport, they are very important in our daily lives From safer water to cleaner clothes • If we pass chlorine gas through cold sodium hydroxide we form sodium chlorate(I): 2NaOH + Cl2 NaCl + NaClO + H2O • NaClO is the active chemical in bleach • As both reactants are made by electrolysis, bleach is often made on the same site • 12% NaClO is used to kill bacteria in water purifying plants • 5% NaClO is used in household bleach • Bleach is an oxidising agent and breaks bonds in the coloured chemicals to form ones which are colourless

12. ES4.1 “Testing bleaches” • Chlorine has a poor public image; • Cl2 and phosgene (COCl2) were used in WW1 • Organochlorine pesticides have polluted the land • Chlorofluorocarbons (CFCs) have polluted the atmosphere • However it is used in many ways that improve our quality of life; • Water treatment to kill bacteria, etc… • Plastics such as PVC • Manufacture of polyurethane • Solvents for removing grease from anything from clothes to metal • Assignment 6 • CI3.1 “Why are bonds like bears?” • CI5.3 “Intermolecular forces” • ES4.2 “What do halogens look like?”

13. Uses of other halogens • Fluorine • First large scale use was to make UF6 for the Manhattan project which developed the atomic bomb • So reactive it is almost impossible to store • Has to be made and then used immediately • Used to make; • sodium fluoride (NaF) which is added to toothpaste and some water supplies • HCFCs for refrigeration • Teflon (PTFE) • Iodine • Dark grey solid, when heated it sublimes to form a purple vapour • Obtained from kelp (seaweed) • Uses: • Antiseptic • Needed in our bodies to produce hormones in our thyroid gland

14. Bromine • Liquid at rtp • Toxic and makes skin blister if spilt • Transported in lead-lined steel tanks • ES4.3 “This liquid is dangerous” • Used for; • Flame retardants • Silver bromide (AgBr) is used in photographic film • Medicines • Dyes • Pesticides (e.g. bromomethane) (although it does also damage the ozone layer) • ES4.4 “Reactions of halogens and halides” • ES4.5 “Check your knowledge (part 1)”

15. ES5 Hydrochloric Acid – an industrial success • HCl is often a secondary product. • A plant makes a particular chemical and a by-product is also made… • …this by-product is then used to make HCl (g) … • …which is then used to make HCl (aq) (hydrochloric acid) • A good example of this is at an electrolysis plant; H2 + Cl2 2HCl • CI15.7 “Percentage yield and atom economy” • It is then dissolved in water to make hydrochloric acid at a concentration of about 30% by mass • It is cheaper to transport it conc. and dilute it at its destination • ES5.1 “Finding the concentration of an acid solution” • Assignment 8 • ES5.2 “Manufacturing halogens and their compounds” • HCl (g) (is made of covalent molecules but dissolves in water to form hydrated ions H+ and Cl-

16. ES6 Treasures of the sea • Everything from bacteria to humans produce organohalide compounds • To date chemists have recorded 4519 unique, naturally occurring ones. Examples include… • Epibatidine which is 500 times more potent than morphine – produced by a frog the size of your finger nail • Many sponges, corals and seaweed use organohalide compounds as part of their defence • Some sea slugs synthesise brominated compounds such as the bitter-tasting panacene to discourage predators

17. Halogenoalkanes • These consist of an alkane with one or more halogen atoms in the place of hydrogen atoms • They are made in the oceans (e.g. seaweeds make CH3Br), by vegetation and in forest fires • Halogenoalkanes can then react in a number of ways… • CH3Br can react with Cl- ions in sea water • One halogen can substitute for another… 2CH3Cl + I2 2CH3I + Cl2 • These are often radical substitution reactions • They can react with water… CH3Cl + H2O CH3OH + HCl • This is a hydrolysis reaction

18. During the 20th century halogenated compounds were used as refrigerants, aerosol propellants and solvents • Many of these were CFCs (chlorofluorocarbons) • The use of these is now being phased out as a result of their impact on the atmosphere (see next topic “Atmosphere”) • CI13.1 “Halogenoalkanes” • ES6.1 “Nucleophilic substitution mechanism” • The ease of making a halogenoalkane depends on which halogens and how many you want to use • ES6.2 “Halogenoalkanes reactivity” • Halogenoalkanes can be made by reacting an alcohol with the appropriate hydrogen halide • This is an example of a substitution reaction

19. ES6.3 “Making a halogenoalkane” • Small amount of chloromethane is also made by reacting methane with chlorine. • This produces four products with different numbers of chlorine atoms joined to the carbon • The mixture is then separated by fractional distillation • Main use nowadays is in manufacture of polymers such as PTFE (teflon) • Assignment 9